The Two Main Reasons Your Ducts Don’t Move Enough Air

Understanding the physics of air movement can lead to better duct systems

This flex duct with the inner liner not pulled tight will have a lot higher resistance to air flow than a flex duct with the inner liner pulled tight.

Two things. Just two things in your ducts are responsible for giving the blower in your furnace or air handler a hard time. They make the blower push against more pressure, thus reducing air flow or increasing energy use, depending on blower type. They cut the amount of air that gets delivered to the rooms. And they can be reduced but not eliminated. Do you know what they are?

Those things are related, but we need to go back further. We want the root causes. This is basic physics I'm talking about. Maybe looking at the image above, a view through a piece of flaccid flex duct liner, will give you an idea of what's to blame.

Friction

The first cause of reduced air flow is friction. When air moving through a duct rubs against the inner surfaces of that duct, it loses energy. It slows down. Its pressure drops. The more it rubs, the more those things happen. It's like walking down a busy sidewalk with your shoulder rubbing against the buildings.

The amount of friction depends on the nature of the material the duct is made of, how it was installed, how dirty it is, and how fast the air is moving. The photo above shows flex duct that's not pulled tight at all. Even though you can't see it all that well, you can tell that there's probably going to be a lot of rubbing when air moves through that duct. The same flex duct pulled tight is shown below. It still looks a bit rough but is much better than the one above. A piece of rigid metal duct would provide a much smoother surface.

Turbulence

The other primary cause of reduced air flow is turbulence. This one is a kind of friction of the air rubbing against itself. The main cause of turbulence within ducts is turning the air. When you send air through a 90° turn, the type of fitting you use to do so can make a big difference.

The diagram below is from ACCA's booklet Understanding the Friction Chart. In both of the 90° elbows, the air enters nice and smoothly. That's laminar flow. When it makes the turn, however, notice that the air in the elbow with the curved inside edge (the throat) results in less turbulence. The elbow with the square throat produces more turbulence. Pick your fittings carefully!

Friction rates and pressure drops

The result of friction and turbulence, as I said above, is that you get a drop in the pressure. As air moves through a supply duct, the pressure created by the fan behind it keeps it moving. The farther it travels down the duct, though, the more that pressure is reduced by friction and turbulence. That's true in good duct systems as well as bad.

Both of these causes, friction and turbulence, are included in the friction rates given for various types of ducts and fittings. As the word “rate” indicates, the friction rate doesn't tell the whole story. You've got to combine it with something else to figure out what the whole pressure drop is. That's where equivalent length comes in, and I'll save that for a future article. Or you can skip ahead and go read Manual D.

When designing and installing ducts, you've got to know about this stuff. To get the best air flow, you want ducts with the least amount of friction and turbulence you can get. To do so, choose low friction materials, like rigid metal, where possible, or make sure you install higher friction ducts like flex as well as you possibly can. Pull the inner liners tight!

To reduce turbulence, rule number one is: Don’t turn the air with flex duct. If you have to do that, make it as big and gradual a turn as possible with the inner liner pulled tight. Better is to turn the air with metal fittings. Best is to turn the air with metal fittings that have round throats or turning vanes.

Friction and turbulence play a big role in whether a duct system does what it's supposed to or not. We've got this stuff quantified. If you're not using Manual D or a ductulator or some other method that quantifies these effects, you may well end up with a system that no amount of commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency.
can save.

About the Authors

Peter Yost is the Director of Residential Services for BuildingGreen, LLC in Brattleboro, Vermont. He has been building, researching, teaching, writing, and consulting on high performance homes for more than twenty years. Read more...

Joseph Lstiburek is a principal of Building Science Corporation. He is a forensic engineer who investigates building failures and is internationally recognized as an authority on moisture related building problems and indoor air quality. Read more...

John Straube, Ph.D., P.Eng., is a principal of Building Science Consulting In Waterloo, Ontario, and a professor of building science in the Civil Engineering Department and School of Architecture at the University of Waterloo, Canada. Dr. Straube has acted as an educator, researcher, consultant and expert witness on energy efficiency, durability and IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness.. Current interests include the optimal system design of buildings, sustainable buildings, and moisture problem avoidance.

Allison Bailes III has a PhD in physics. He is also a RESNET-accredited energy consultant, trainer, and the principal of Energy Vanguard, a consulting firm in Decatur, Georgia.